1. Field of the Invention
The present invention relates to a method of producing a compressible layer to be utilized for a blanket for printing operation as well as a method of producing a blanket for printing operation with such compressible layer.
2. Prior Art
As is widely known, a blanket in use for printing operation such as high speed offset printing or the like is often provided with a compressible layer made of polymer having a certain elasticity. Firstly, this compressible layer is intended to uniformly absorb the expansion caused on the compression surface of the blanket under the influence of the force exerted thereon during the printing operation and thereby reduce deformation of the blanket at the surface to obtain clear and beautiful printed letters, patterns, and the like. The compressed layer also is intended to absorb or damp the extra force exerted on a plate cylinder and the surface of the blanket (the extra force being exerted thereon in addition to normal printing force) when a printing paper is delivered in the folded state or two or more printing papers are incorrectly delivered in the superimposed state to a printing machine so that the running life of both the plate cylinder and the blanket is remarkably elongated. Thirdly, the compressible layer is further intended to assure that clear and beautiful printed letters and patterns can be obtained irrespective of skillfulness of an operator even when the cylinder arrangement is conducted to an apperciable excessive extent over the standard cylinder arrangement, although the regular blanket without any compressive layer employed therefor is required to carry out strict controlling operation with respect to its thickness so as to properly conduct the predetermined cylinder arrangement.
There have been already proposed a number of methods of producing a compressible layer of the above-mentioned type. One of them is a method of producing a compressible layer with a number of voids included therein by the steps of adding foaming agent to synthetic rubber and causing foaming phenomenon to take place by heating during vulcanization of the synthetic rubber (as disclosed in Japanese Patent Publication No. 3045/74). The other methods include the step of causing elastomer to mix into a porous felt (as disclosed in Japanese Patent Publication No. 131110/62) and the steps of dispersing a number of microcapsules into elastomer and producing a closed cell later (as disclosed in Japanese Patent Publication No. 7371/77).
As will be readily understood from the above description, it is required that a compressible layer utilized in a blanket for printing operation has the following functional characteristics:
1. Uniform absorption of the expansion caused on the surface of a blanket for printing operation.
2. Effective damping of the extra force exerted on the surface of the blanket.
3. Restorability of compression stress at a speed responsive to the working speed during the printing operation at a high speed.
4. Long durability.
Further, it is also required that a method of producing a compressible layer is adaptable to any variation, because the functional characteristics required for the blanket for printing operation vary in dependence on printing process employed and other factors.
Moreover, to meet the requirement with respect to the above-mentioned functional characteristics, a compressible layer is required to have compression stress in the range of 2.0 to 8.0 Kg/cm.sup.2 in terms of compressibility (the compression stress is referred to under the working conditions of the compression speed of lmm/min and the compression distance of 0.15 mm) and peeling strength 200 g/cm or higher (as measured in conformance with the standard peeling test), wherein the compression stress is selectively determined within the range of 2.0 to 8.0 Kg/cm.sup.2.
However, it has been found that any of the conventional methods fails to produce a compressible layer having the compression stress lower than 4 Kg/cm.sup.2 ; therefore, it fails to produce a compressible layer utilized in a blanket for printing operation having excellent compressibility.